Medical fluid cassettes and related systems and methods

ABSTRACT

This disclosure relates to medical fluid cassettes and related systems and methods. In certain aspects, a medical fluid cassette includes a base having a first region and a second region, a first membrane overlying the first region of the base, and a second membrane overlying the second region of the base. The second membrane is configured to rebound away from the base when a force used to press the second membrane toward the base is released.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of and claims priority under 35 U.S.C.§ 120 to U.S. Ser. No. 13/492,370, filed Jun. 8, 2012. The contents ofthis priority application are hereby incorporated by reference in itsentirety.

TECHNICAL FIELD

This disclosure relates to medical fluid cassettes and related systemsand methods.

BACKGROUND

Dialysis is a treatment used to support a patient with insufficientrenal function. The two principal dialysis methods are hemodialysis andperitoneal dialysis.

During hemodialysis (“HD”), the patient's blood is passed through adialyzer of a dialysis machine while also passing a dialysis solution ordialysate through the dialyzer. A semi-permeable membrane in thedialyzer separates the blood from the dialysate within the dialyzer andallows diffusion and osmosis exchanges to take place between thedialysate and the blood stream. These exchanges across the membraneresult in the removal of waste products, including solutes like urea andcreatinine, from the blood. These exchanges also regulate the levels ofother substances, such as sodium and water, in the blood. In this way,the dialysis machine acts as an artificial kidney for cleansing theblood.

During peritoneal dialysis (“PD”), a patient's peritoneal cavity isperiodically infused with dialysis solution or dialysate. The membranouslining of the patient's peritoneum acts as a natural semi-permeablemembrane that allows diffusion and osmosis exchanges to take placebetween the solution and the blood stream. These exchanges across thepatient's peritoneum, like the continuous exchange across the dialyzerin HD, result in the removal waste products, including solutes like ureaand creatinine, from the blood, and regulate the levels of othersubstances, such as sodium and water, in the blood.

Many PD machines are designed to automatically infuse, dwell, and draindialysate to and from the patient's peritoneal cavity. The treatmenttypically lasts for several hours, often beginning with an initial draincycle to empty the peritoneal cavity of used or spent dialysate. Thesequence then proceeds through the succession of fill, dwell, and drainphases that follow one after the other. Each phase is called a cycle.

SUMMARY

In one aspect of the invention, a medical fluid pumping system includesa medical fluid pumping machine defining a cassette enclosure andincluding a movable piston. The system further includes a medical fluidcassette configured to be disposed within the cassette enclosure of themedical fluid pumping machine. The medical fluid cassette includes abase having a first region and a second region, a first membraneoverlying the first region of the base, and a second membrane overlyingthe second region of the base. The first membrane cooperates with thefirst region of the base to form at least one fluid pathway and thesecond membrane cooperates with the second region of the base to definea fluid pump chamber. The second membrane is more resilient than thefirst membrane. The cassette is positionable within the cassetteenclosure of the medical fluid pumping machine so that the secondmembrane can be moved toward the base by the piston to decrease a volumeof the fluid pump chamber and, upon retraction of the piston, the secondmembrane can rebound to increase the volume of the pump chamber.

In another aspect of the invention, a medical fluid cassette includes abase having a first region and a second region, a first membraneoverlying the first region of the base, and a second membrane overlyingthe second region of the base. The first membrane cooperates with thefirst region of the base to form at least one fluid pathway and thesecond membrane cooperates with the second region of the base to definea fluid pump chamber. The second membrane is more resilient than thefirst membrane and is configured to rebound away from the base when aforce used to press the second membrane toward the base is released.

In an additional aspect of the invention, a medical fluid deliverymethod includes expelling a medical fluid from a fluid pump chamberdefined between a membrane and a recessed region of a base of a medicalfluid cassette by using a piston to press the membrane into the recessedregion, and then drawing medical fluid into the fluid pump chamber byretracting the piston and allowing the membrane to rebound toward theretracting piston, wherein the membrane is allowed to rebound toward theretracting piston head due to resiliency of the membrane.

In a further aspect of the invention, a medical fluid pumping machineincludes a housing that at least partially defines a cassette enclosureconfigured to receive a medical fluid cassette and a piston that istranslatable relative to the housing. The cassette includes a fluid pumpchamber defined between a membrane and a base, and the piston includes apiston head having a circumferential region that is configured to moveradially inward as the piston head is pressed against the membrane ofthe cassette to move the membrane toward a base of the cassette.

Implementations can include one or more of the following features.

In some implementations, the second region of the base is a recessedregion of the base.

In certain implementations, the first region of the base is asubstantially planar region of the base from which a plurality of raisedridges extend.

In some implementations, the second membrane has a greater thicknessthan the first membrane.

In certain implementations, the first membrane has a thickness of about0.004 inch to about 0.006 inch, and the second membrane has a thicknessof about 0.05 inch to about 0.12 inch.

In some implementations, the first and second membranes are formed ofdifferent materials.

In certain implementations, the second membrane is formed of anelastomer.

In some implementations, the second region of the base is a recessedregion of the base, and the second membrane is sized to overlie aportion of the base that surrounds the recessed region.

In certain implementations, a fluid-tight seal is formed between aperipheral region of the second membrane and the base.

In some implementations, the second membrane includes a dome-shapedportion.

In certain implementations, the medical fluid cassette further includesa first ring that is secured to the second membrane and compresses thesecond membrane against the base.

In some implementations, the first ring is fastened to the base.

In certain implementations, the medical fluid pumping system furtherincludes a second ring disposed on a side of the base opposite thesecond membrane, and the second ring is secured to the first ring in amanner to cause the first ring to compress the second membrane againstthe base.

In some implementations, the first and second rings are secured to thebase by mechanical fasteners.

In certain implementations, the second membrane is attached to theportion of the base that surrounds the recessed region.

In some implementations, the second membrane is welded to the portion ofthe base that surrounds the recessed region.

In certain implementations, the first membrane defines an aperture thatis aligned with the second region of the base, and the second membraneis at least partially disposed within the aperture of the firstmembrane.

In some implementations, the first membrane covers substantially theentire surface of the base.

In certain implementations, the first membrane is attached to the basein a perimeter region of the base.

In some implementations, the second membrane is attached (e.g.,adhesively attached) to a portion of the first membrane that overliesthe second region of the base.

In certain implementations, the second membrane overlies substantiallythe entire surface of the base.

In some implementations, the second membrane includes a plurality ofcutouts that are aligned with valve regions of the cassette.

In certain implementations, the second membrane is configured to createa vacuum pressure of about 150 mbar to about 200 mbar within the fluidpump chamber when the second membrane is allowed to rebound after beingpressed toward the base (e.g., by the piston).

In some implementations, the piston includes a piston head having acircumferential region that is configured to move radially inward as thepiston head is pressed against the second membrane to move the secondmembrane toward the base.

In certain implementations, the circumferential region of the pistonhead is formed of an elastomeric material that compresses as the pistonhead is pressed against the second membrane to move the second membranetoward the base.

In some implementations, the piston head includes a plurality ofinterleaved segments that move relative to one another to allow thecircumferential region of the piston head to collapse as the piston headis pressed against the second membrane to move the second membranetoward the base.

In certain implementations, the interleaved segments are in the form ofleaves.

In some implementations, the interleaved segments are spring-loaded tobias the piston head to an expanded position.

In certain implementations, the piston head includes a plurality oftelescoping segments that move relative to one another to allow thecircumferential region of the piston head to collapse as the piston headis pressed against the second membrane to move the second membranetoward the base.

In some implementations, the telescoping segments are rings.

In certain implementations, the telescoping segments are secured to aspring that biases the piston head to a flat configuration.

In some implementations, at any given time throughout an outward strokeof the piston, an area of a portion of the piston head in contact withthe second membrane is substantially equal to an area of the pumpchamber in a plane in which the second membrane lies.

In certain implementations, the medical fluid pumping system is adialysis system (e.g., a peritoneal dialysis system).

In some implementations, the medical fluid cassette is a dialysis fluidcassette (e.g., a peritoneal dialysis fluid cassette).

In certain implementations, the membrane creates a vacuum pressure ofabout 150 mbar to about 200 mbar within the fluid pump chamber when themembrane is allowed to rebound after being pressed into the recessedregion by the piston.

In some implementations, the piston includes a piston head having acircumferential region that is configured to move radially inward as thepiston head is pressed against the membrane to press the membrane intothe recessed region.

In certain implementations, at any given time throughout the retractionof the piston, an area of a portion of the piston in contact with themembrane is substantially equal to an area of the pump chamber in aplane in which the membrane lies.

In some implementations, the medical fluid is dialysate.

In certain implementations, the circumferential region of the pistonhead is formed of an elastomeric material that compresses as the pistonhead is pressed against the membrane to move the membrane toward thebase.

In some implementations, the piston head includes a plurality ofinterleaved segments that move relative to one another to allow thecircumferential region of the piston head to collapse as the piston headis pressed against the membrane to move the membrane toward the base.

In certain implementations, the interleaved segments are in the form ofleaves.

In some implementations, the interleaved segments are spring-loaded tobias the piston head to an expanded position.

In certain implementations, the piston head includes a plurality oftelescoping segments that move relative to one another to allow thecircumferential region of the piston head to collapse as the piston headis pressed against the membrane to move the membrane toward the base.

In some implementations, the telescoping segments are rings.

In certain implementations, the telescoping segments are secured to aspring that biases the piston head to a flat configuration.

In some implementations, at any given time throughout an outward strokeof the piston, an area of a portion of the piston head in contact withthe membrane is substantially equal to an area of the pump chamber in aplane in which the membrane lies.

In certain implementations, the medical fluid pumping machine furtherincludes a door secured to the housing, and the door and the housingcooperate to define the cassette enclosure when the door is closed.

Implementations can include one or more of the following advantages.

In some implementations, the membrane, after being pressed toward theregion of the base that partially defines the pump chamber, rebounds asa result of its own internal forces (or self-expands) with sufficientforce to create suction within the pump chamber that draws fluid intothe pump chamber. Thus, fluid can be drawn into the pump chamber withoutrequiring the membrane to be coupled (e.g., via vacuum pressure,adhesive, or mechanical fasteners) to the piston of the medical fluidpumping machine.

In certain implementations, the design of the membrane allows themedical fluid pumping system to be operated without permanentlydeforming (e.g., stretching) the membrane. For example, the membrane canbe thicker than many conventional cassette membranes and/or can have adome-shaped region that allows the membrane to be fully deflected intothe recessed region of the base and then to rebound without permanentdeformation. As a result, the pumping volume accuracy of the system canbe improved as compared to conventional systems that utilize cassetteshaving thinner, flat membranes that permanently deform or stretch duringuse.

In some implementations, the portions of the membrane overlying the pumpchamber are substantially prevented from bulging outward (i.e., awayfrom the base of the cassette) during the fluid pumping process. In someimplementations, for example, the membrane is thicker than manyconventional cassette membranes. This construction enables the portionsof the membrane that overlie the pump chamber but are not in contactwith the piston as the piston is advanced toward the base of thecassette to withstand the increased fluid pressure within the pumpchamber without bulging outward.

In certain implementations, a piston head of the piston is designed sothat the area of the piston head that is in contact with the membraneoverlying the pump chamber throughout an outward stroke of the piston(i.e., as the piston is advanced towards the cassette base) issubstantially equal to the area of the pump chamber in the plane inwhich the membrane lies. For example, the piston head can have acircumferential region that is compressible or collapsible such that thepressure applied to the piston head by the membrane as the piston headis advanced (and as the area of the pump chamber gradually decreases)causes the circumferential region to compress or collapse. This pistonhead design can prevent or significantly reduce outward bulging of thecassette membrane as a result of increased fluid pressure in the pumpchamber because the piston head contacts and resists outward bulging inthose areas of the membrane that tend to bulge outwardly in manyconvention medical fluid cassettes.

Other aspects, features, and advantages will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a peritoneal dialysis (“PD”) system thatincludes a PD cycler positioned atop a portable cart.

FIG. 2 is a perspective view of the PD cycler and PD cassette of the PDsystem of FIG. 1. A door of the PD cycler is in the open position toshow the inner surfaces of the PD cycler that interface with the PDcassette during use.

FIG. 3 is a perspective view of an open cassette compartment of the PDcycler of FIGS. 1 and 2.

FIG. 4 is an exploded, perspective view of the PD cassette of the PDsystem of FIG. 1.

FIG. 5 is a perspective view of the PD cassette, from a membrane side ofthe PD cassette.

FIG. 6 is a perspective view of the PD cassette, from a rigid base sideof the PD cassette.

FIG. 7 is a side view of the PD cassette.

FIG. 8 is a cross-sectional view of the PD cassette, taken along line8-8 in FIG. 6.

FIG. 9 is a partial perspective view of the PD system of FIG. 1 with thePD cassette in the cassette compartment of the PD cycler and the door ofthe PD cycler open.

FIGS. 10A-10C are diagrammatic cross-sectional views of the PD system ofFIG. 1 with the PD cassette in the cassette compartment of the PDcycler, during different phases of operation.

FIGS. 11A-11C illustrate various fluid flow paths through the PDcassette of the PD system of FIG. 1 during a PD treatment.

FIGS. 12 and 13 are perspective views from a membrane side and from arigid base side, respectively, of a PD cassette that includes pumpingmembranes and a support ring that is separate from the pumping membranesand that cooperates with a retainer ring on the opposite rigid base sideof the PD cassette to secure the pumping membranes to a base of the PDcassette.

FIG. 14 is a perspective view of a PD cassette that includessubstantially flat pumping membranes that overlie pump chambers of thePD cassette and are directly attached to a base of the PD cassette.

FIG. 15 is a cross-sectional view of a piston including a piston headthat has a compressible portion that surrounds a central core.

FIGS. 16A-16D are diagrammatic cross-sectional views of the PD cassetteof FIG. 14 in the cassette compartment of a PD cycler equipped with thepiston of FIG. 15, during different phases of operation.

FIG. 17 is a cross-sectional view of a piston including a piston headformed of an elastomeric material that generally decreases in thicknessfrom a central region of the piston head toward a periphery of thepiston head.

FIGS. 18A and 18B are cross-sectional views of a flat configuration anda deformed configuration, respectively, of a piston having a piston headthat includes multiple concentric rings secured to a leaf spring.

FIGS. 19A and 19B are side views of a piston having a piston head thatincludes multiple overlapping leaves, an expanded configuration and acompressed configuration, respectively.

DETAILED DESCRIPTION

In certain aspects of the invention, a medical fluid cassette (e.g., adialysis fluid cassette) includes a relatively thick membrane thatoverlies a recessed region of a base to form a fluid pump chamber.During use, a piston of a medical fluid pumping machine (e.g., adialysis machine) presses against the membrane to move the membranetoward the base and expel fluid from the fluid pump chamber. In somecases, the piston has a piston head with a circumferential region thatcompresses or collapses as the piston moves the membrane into therecessed region of the base. Such a configuration can help to ensurethat uniform pressure is applied to the membrane by the piston headthroughout the outward stroke of the piston. The piston is subsequentlyretracted and the membrane rebounds under its own force (orself-expands) causing fluid to be drawn into the fluid pump chamber.Examples of medical fluid cassettes and medical fluid pumping machinesare described below.

Referring to FIG. 1, a peritoneal dialysis (“PD”) system 100 includes aPD cycler (also referred to as a PD machine) 102 seated on a cart 104.Referring also to FIG. 2, the PD cycler 102 includes a housing 106, adoor 108, and a cassette interface 110 that mates with a disposable PDcassette 112 when the cassette 112 is disposed within a cassettecompartment 114 formed between the cassette interface 110 and the closeddoor 108. A heater tray 116 is positioned on top of the housing 106. Theheater tray 116 is sized and shaped to accommodate a bag of dialysissolution (e.g., a 5 liter bag of dialysis solution). The PD cycler 102also includes a touch screen 118 and additional control buttons 120 thatcan be operated by a user (e.g., a patient) to allow, for example,set-up, initiation, and/or termination of a PD treatment.

Dialysis solution bags 122 are suspended from fingers on the sides ofthe cart 104, and a heater bag 124 is positioned on the heater tray 116.The dialysis solution bags 122 and the heater bag 124 are connected tothe cassette 112 via dialysis solution bag lines 126 and a heater bagline 128, respectively. The dialysis solution bag lines 126 can be usedto pass dialysis solution from dialysis solution bags 122 to thecassette 112 during use, and the heater bag line 128 can be used to passdialysis solution back and forth between the cassette 112 and the heaterbag 124 during use. In addition, a patient line 130 and a drain line 132are connected to the cassette 112. The patient line 130 can be connectedto a patient's abdomen via a catheter and can be used to pass dialysissolution back and forth between the cassette 112 and the patient duringuse. The drain line 132 can be connected to a drain or drain receptacleand can be used to pass dialysis solution from the cassette 112 to thedrain or drain receptacle during use.

FIG. 3 shows a more detailed view of the cassette interface 110 and thedoor 108 of the PD cycler 102. As shown, the PD cycler 102 includespistons 135A, 135B with substantially hemispherical piston heads 134A,134B that can be axially moved within piston access ports 136A, 136Bformed in the cassette interface 110. The piston heads 134A, 134B can beformed of any of various different polymers, metals, and/or alloys. Insome implementations, the piston heads 134A, 134B are made ofpolyoxymethylene (marketed under the trade name Delrin available fromDupont of Wilmington, Del.). The hemispherical shape of the piston heads134A, 134B can be achieved using any of various different techniques,including machining techniques molding techniques, and/or castingtechniques.

The pistons 135A, 135B include piston shafts 133A, 133B (shown in FIGS.10A-10C) that are coupled to motors that can be operated to move thepiston heads 134A, 134B axially inward and outward within the pistonaccess ports 136A, 136B. As discussed below, when the cassette 112(shown in FIGS. 2 and 4-8) is positioned within the cassette compartment114 with the door 108 closed, the piston heads 134A, 134B of the PDcycler 102 align with pump chambers 138A, 138B of the cassette 112. As aresult, the piston heads 134A, 134B can be moved in the direction of thecassette 112 to force pumping membranes 161A, 161B towards a rigid base156 of the cassette 112, causing the volume defined by the pump chambers138A, 138B to decrease and forcing dialysis solution out of the pumpchambers 138A, 138B. The piston heads 134A, 134B can also be retractedaway from the cassette 112 and out of the volume defined by the pumpchambers 138A, 138B. As discussed in greater detail below, the pumpingmembranes 161A, 161B are resilient members that automatically rebound(or self-expand) toward the piston heads 134A, 134B as the piston heads134A, 134B are retracted. As a result, the volume defined by the pumpchambers 138A, 138B increases and dialysis solution is drawn into thepump chambers 138A, 138B as the piston heads 134A, 134B retract and thepumping membranes 161A, 161B are allowed to rebound.

Referring again to FIG. 3, the PD cycler 102 also includes multipleinflatable members 142 positioned within inflatable member access ports144 in the cassette interface 110. The inflatable members 142 align withdepressible dome regions 146 of the cassette 112 when the cassette 112is positioned within the cassette compartment 114. While only one of theinflatable members 142 is labeled in FIG. 3, it should be understoodthat the PD cycler 102 includes an inflatable member associated witheach of the depressible dome regions 146 of the cassette 112 (shown inFIG. 5). The inflatable members 142 act as valves to direct dialysissolution through the cassette 112 in a desired manner during use. Inparticular, the inflatable members 142 bulge outward beyond the surfaceof the cassette interface 110 and into contact with the depressible domeregions 146 of the cassette 112 when inflated, and retract into theinflatable member access ports 144 and out of contact with the cassette112 when deflated. By inflating some of the inflatable members 142 anddeflating other inflatable members 142, certain fluid flow paths withinthe cassette 112 will be blocked off while other fluid flow paths withinthe cassette 112 will remain open. Thus, dialysis solution can be pumpedthrough the cassette 112 by actuating the piston heads 134A, 134B, andcan be guided along desired flow paths within the cassette 112 byselectively inflating and deflating the inflatable members 142.

Still referring to FIG. 3, locating pins 148 extend from the cassetteinterface 110. When the door 108 is in the open position, the cassette112 can be loaded onto the cassette interface 110 by positioning the topportion of the cassette 112 under the locating pins 148 and pushing thebottom portion of the cassette 112 toward the cassette interface 110.The cassette 112 is dimensioned to remain securely positioned betweenthe locating pins 148 and a lower ledge 150 extending from the cassetteinterface 110 to allow the door 108 to be closed over the cassette 112.The locating pins 148 help to ensure that the pump chambers 138A, 138Bof the cassette 112 are aligned with the piston heads 134A, 134B whenthe cassette 112 is positioned in the cassette compartment 114 betweenthe closed door 108 and the cassette interface 110.

The door 108, as shown in FIG. 3, defines recesses 152A, 152B thatsubstantially align with the piston heads 134A, 134B when the door 108is in the closed position. When the cassette 112 is positioned withinthe cassette compartment 114, hollow projections 154A, 154B of thecassette 112 (shown in FIGS. 6-8), inner surfaces of which cooperatewith the membrane 140 to form the pump chambers 138A, 138B, fit withinthe recesses 152A, 152B. The door 108 further includes a pad that can beinflated during use to compress the cassette 112 between the door 108and the cassette interface 110. With the pad inflated, the portions ofthe door 108 forming the recesses 152A, 152B support the projections154A, 154B and the planar surface of the door 108 supports the otherregions of the cassette 112. The door 108 can counteract the forcesapplied by the piston heads 134A, 134B and the inflatable members 142and thus allows the piston heads 134A, 134B to depress the portions ofthe membrane 140 overlying the pump chambers 138A, 138B and similarlyallows the inflatable members 142 to actuate the depressible domeregions 146 on the cassette 112.

The PD cycler 102 includes various other features not described indetail herein. Further details regarding the PD cycler 102 and itsvarious components can be found in U.S. Patent Application PublicationNo. 2007/0112297, which is incorporated by reference herein.

FIG. 4 is an exploded, perspective view of the cassette 112. FIGS. 5 and6 are perspective views of the assembled cassette 112 from the membraneside and rigid base side, respectively, and FIG. 7 is a side view of theassembled cassette 112. Referring to FIGS. 4-6, the pumping membranes161A, 161B, which partially encapsulate support rings 169A, 169B,overlie recessed regions 163A, 163B formed by the hollow dome-shapedprotrusions 154A, 154B (shown in FIGS. 6-8) of the base 156. Peripheralregions 177A, 177B of the pumping membranes 161A, 161B extend into therecessed regions 163A, 163B of the base 156 and engage the inner wallsof the hollow dome-shaped protrusions 154A, 154B. Retainer rings 171A,171B are positioned on the rigid base side of the cassette 112 oppositethe pumping membranes 161A, 161B and surround the hollow dome-shapedprotrusions 154A, 154B of the base 156. Nuts 173A, 173B and bolts 175A,175B are used to apply a compressive force to peripheral regions of thepumping membranes 161A, 161B when the cassette is fully assembled, aswill be described in greater detail below. As a result, the peripheralregions of the pumping membranes 161A, 161B are pressed against the base156 to form a liquid-tight seal around the recessed regions 163A, 163Bwhen the cassette 112 is fully assembled.

In addition to the pumping membranes 161A, 161B, the cassette 112includes a thinner, flexible membrane 140 that is attached to theperiphery of the base 156 and to planar portions of the base 156 thatsurround recessed regions 163A, 163B. The flexible membrane 140 includesopenings 141A, 141B that are sized and shaped to generally correspond tothe recessed regions 163A, 163B and to receive the pumping membranes161A, 161B. A series of raised ridges 167 extend from a planar surfaceof the base 156 towards and into contact with the inner surface of theflexible membrane 140 to form fluid pathways that lead to and from thepump chambers 138A, 138B when the cassette 112 is compressed between thedoor 108 and the cassette interface 110 of the PD cycler 102.

Referring now to FIGS. 4 and 8, the pumping membranes 161A, 161B extendpartially into the recessed regions 163A, 163B of the base 156 andcooperate with the recessed regions 163A, 163B of the base 156 to formthe fluid pump chambers 138A, 138B. The relatively thick peripheralregions 177A, 177B of the pumping membranes 161A, 161B, which partiallyencapsulate the support rings 169A, 169B, engage the circumferences ofthe recessed regions 163A, 163B of the base 156 to form liquid-tightseals around the circumferences of the pump chambers 138A, 138B. Theperipheral regions 177A, 177B of the pumping membranes 161A, 161B definebores that align with bores extending through the support rings 169A,169B. The bores of the pumping membranes 161A, 161B and support rings169A, 169B align with bores formed in the hollow protrusions 154A, 154Bof the cassette base 156 to form through-holes through which the bolts175A, 175B can extend.

Dome-shaped central portions 179A, 179B of the pumping membranes 161A,161B overlie the associated central portions of the recessed regions163A, 163B of the base 156 to form the pump chambers 138A, 138B. Thedistance between the apexes of the dome-shaped portions 179A, 179B andthe apexes of the associated hollow protrusions 154A, 154B typicallyranges from about 0.100 inch to about 0.5 inch. The dome-shaped portions179A, 179B of the pumping membranes 161A, 161B provide increased pumpchamber volume, as compared to cassettes that have flat or planarpumping membranes. In addition, the dome-shaped portions 179A, 179B ofthe pumping membranes 161A, 161B facilitate rebounding of the pumpingmembranes 161A, 161B away from the base 156 as the pistons are retractedduring use. It is this rebounding action of the pumping membranes 161A,161B that generates vacuum pressure within the fluid pump chambers 138A,138B, allow dialysis solution to be pulled into the fluid pump chambers138A, 138B.

The dome-shaped central portions 179A, 179B of the pumping membranes161A, 161B are thinner (e.g., about 0.05 inch to about 0.20 inchthinner) than the peripheral regions 177A, 177B of the pumping membranes161A, 161B. For example, each of the dome-shaped portions 179A, 179B canhave a thickness, measured at the top or apex of the dome-shaped portion179A, 179B, of about 0.05 inch to about 0.10 inch, and each of theperipheral regions 177A, 177B can have a thickness of about 0.120 inchto about 0.250 inch in their edge regions. In some implementations, thedome-shaped portions have a thickness of about 0.075 inch and theperipheral regions 177A, 177B have a thickness of about 0.120 inch. Theincreased thickness of the pumping membranes 161A, 161B in theperipheral regions 177A, 177B can provide increased resilience to thedome-shaped portions 179A, 179B and thus increase the ability of thedome-shaped portions 179A, 179B to self-expand after being compressedinwardly toward the base 156.

The material and shape of the pumping membranes 161A, 161B can beselected to provide the pumping membranes 161A, 161B with a desiredresiliency. In certain implementations, the pumping membranes 161A, 161Bare configured to cause the pumping membranes 161A, 161B to self-expandor rebound with an outward force of about 20N to about 250N (e.g., about20N to about 100N, about 55N) after being pressed into the recessedregions 163A, 163B of the base 156 and then released (e.g., by extendingand then retracting the pistons of the PD cycler). By expanding withsuch a force, the pumping membranes 161A, 161B can create a vacuumpressure of about 150 mbar to about 200 mbar (e.g., about 150 mbar)within the pump chambers 138A, 138B and within fluid lines that arefluidly connected to the pump chamber. However, the pumping membranes161A, 161B can be formed in a way to expand with higher or lower forces,depending on the intended use or application of the cassette 112.

Typically, the pumping membranes 161A, 161B are formed of siliconerubber. However, as an alternative to or in addition to silicone rubber,the pumping membrane material can include various other resilientelastomeric materials, such as neoprene, nitrile rubber (e.g., Buna-n),fluoroelastomer (e.g., Viton), etc.

Still referring to FIGS. 4 and 8, the support rings 169A, 169B arepartially encapsulated within the material of the pumping membranes161A, 161B such that only the bottom surfaces of the support rings 169A,169B (based on the view shown in FIG. 8) are exposed. Cavities extendinwardly from these exposed surfaces of the support rings 169A, 169B forreceiving the nuts 173A, 173B. These cavities and sized and shaped sothat the outer surfaces of the nuts 173A, 173B are substantially flushwith the exposed surfaces of the support rings 169A, 169B, which aresubstantially flush with the outers surfaces of the peripheral regions177A, 177B of the pumping membranes 161A, 161B. The support rings 169A,169B include annular projections 181A, 181B that together with thepumping membrane material surrounding those projections 181A, 181Bengage raised annular flanges or lips 165A, 165B that extend from thebase 156 and surround the recessed regions 163A, 163B. This arrangementcan help to prevent the peripheral regions 177A, 177B of the pumpingmembranes 161A, 161B from sliding further into the recessed regions163A, 163B when the pistons 135A, 135B are advanced during use to pushthe dome-shaped portions 179A, 179B of the pumping membranes 161A, 161Binto the recessed regions 163A, 163B to expel dialysis solution from thefluid pump chambers 138A, 138B.

The support rings 169A, 169B are more rigid than the pumping membranematerial and thus provide the peripheral regions 177A, 177B of thepumping membranes 161A, 161B with increased strength. The support rings169A, 169B also distribute forces applied by the nuts 173A, 173B andbolts 175A, 175B across a larger area of the membrane than if the nuts173A, 173B and bolts 175A, 175B were to extend through the pumpingmembrane material alone. As a result, the support rings 169A, 169B canhelp to prevent the peripheral regions 177A, 177B of the pumpingmembranes 161A, 161B from tearing or deforming and can therefore help tomaintain a liquid-tight seal between the pumping membranes 161A, 161Band the cassette base 156.

The size of the support rings 169A, 169B is dependent upon the size ofthe cassette being used and the amount of additional support desired tobe added to the pumping membrane material for a particular application.In some implementations, the support rings 169A, 169B have an innerdiameter of about 1.0 inch to about 2.0, an outer diameter of about 2.0inch to about 3.0 inch, and a thickness of about 0.050 inch to about0.200 inch.

The support rings 169A, 169B are typically formed of polypropylene.However, as an alternative to or in addition to polypropylene, thesupport rings 169A, 169B can include certain other rigid plastics (e.g.,polycarbonate, ABS, polyvinyl chloride, etc.) and/or certain metals(e.g., aluminum, steel, stainless steel, etc.).

The support rings 169A, 169B are typically encapsulated within theelastomeric material of the pumping membranes 161A, 161B using anovermolding technique. However, any of various other techniques thatenable encapsulation or partial encapsulation of the support rings 169A,169B by the membrane material can be used.

Still referring to FIGS. 4 and 8, the retainer rings 171A, 171B surroundthe hollow protrusions 154A, 154B extending from the opposite side ofthe rigid base 156. The retainer rings 171A, 171B include bores thatalign with the bores of the hollow protrusions 154A, 154B, the pumpingmembranes 161A, 161B, and the support rings 169A, 169B such that thebolts 175A, 175B can pass through those components and engage the nuts173A, 173B disposed within the cavities of the support rings 169A, 169B.The nuts 173A, 173B and bolts 175A, 175B are tightened to press thepumping membranes 161A, 161B against the base 156 and form aliquid-tight seal around the pump chambers 138A, 138B. The retainerrings 171A, 171B distribute the forces of the bolts 175A, 175B across alarger surface area of the base 156 than if the bolts 175A, 175B weresecured directly to the base 156. In addition, the retainer rings 171A,171B provide flat surfaces for engaging the flat heads of the bolts175A, 175B.

The size and material of each of the retainer rings 171A, 171B can beselected to provide sufficient support to withstand the compressionforces applied by the nuts 173A, 173B and bolts 175A, 175B withoutdamage. In some implementations, the retainer rings 171A, 171B have aninner diameter of about 1.0 inch to about 2.0, an outer diameter ofabout 2.0 inch to about 3.0 inch, and a thickness of about 0.050 inch toabout 0.200 inch. Typically, the retainer rings 171A, 171B are formed ofpolypropylene. However, as an alternative to or in addition topolypropylene, the retainer rings 171A, 171B can include certain otherrigid plastics (e.g., polycarbonate, ABS, polyvinyl chloride, etc.)and/or certain metals (e.g., aluminum, steel, stainless steel, etc.).

Referring now to FIGS. 4-6, the flexible membrane 140 is attached to theperiphery of the base 156 and to planar portions of the base 156surrounding the recessed regions 163A, 163B. All other portions of themembrane 140 that overlie the base 156 are typically not attached to thebase 156. Rather, those portions of the membrane 140 sit loosely atopthe raised ridges 167 extending from the planar surface of the base 156.Any of various attachment techniques, such as adhesive bonding andthermal bonding, can be used to attach the membrane 140 to the peripheryof the base 156.

When compressed against the base 156, the flexible membrane 140cooperates with the series of raised ridges 167 extending from theplanar surface of the base 156 to form a series of fluid pathways 158that lead to and from the pump chambers 138A, 138B and to form themultiple, depressible dome regions 146, which are widened portions(e.g., substantially circular widened portions) of the fluid pathways158. During use, the dialysis solution flows to and from the pumpchambers 138A, 138B via the fluid pathways 158 and dome regions 146. Ateach depressible dome region 146, the membrane 140 can be deflected tocontact the planar surface of the base 156 from which the raised ridges167 extend. Such contact can substantially impede (e.g., prevent) theflow of dialysis solution along the region of the pathway 158 associatedwith that dome region 146 during use. Thus, as described in furtherdetail below, the flow of dialysis solution through the cassette 112 canbe controlled through the selective depression of the depressible domeregions 146 by selectively inflating the inflatable members 142 of thePD cycler 102.

The thickness and material(s) of the membrane 140 are selected so thatthe membrane 140 has sufficient flexibility to flex toward the base 156in response to the force applied to the membrane 140 by the inflatablemembers 142. In certain implementations, the membrane 140 is about 0.100micron to about 0.150 micron in thickness. However, various otherthicknesses may be sufficient depending on the type of material used toform the membrane 140.

Any of various different materials that permit the membrane 140 todeflect in response to inflation of the inflatable members 142 withouttearing can be used to form the membrane 140. In some implementations,the membrane 140 includes a three-layer laminate. In certainimplementations, for example, inner and outer layers of the laminate areformed of a compound that is made up of 60 percent Septon® 8004thermoplastic rubber (i.e., hydrogenated styrenic block copolymer) and40 percent ethylene, and a middle layer is formed of a compound that ismade up of 25 percent Tuftec® H1062 (SEBS: hydrogenated styrenicthermoplastic elastomer), 40 percent Engage® 8003 polyolefin elastomer(ethylene octene copolymer), and 35 percent Septon® 8004 thermoplasticrubber (i.e., hydrogenated styrenic block copolymer). The membrane canalternatively include more or fewer layers and/or can be formed ofdifferent materials.

The rigidity of the base 156 helps to hold the cassette 112 in placewithin the cassette compartment 114 of the PD cycler 102 and to preventthe base 156 from flexing and deforming in response to forces applied tothe projections 154A, 154B by the piston heads 134A, 134B and inresponse to forces applied to the planar surface of the base 156 by theinflatable members 142. The base 156 can be formed of any of variousrelatively rigid materials. In some implementations, the base 156 isformed of one or more polymers, such as polypropylene, polyvinylchloride, polycarbonate, polysulfone, and other medical grade plasticmaterials. In certain implementations, the base 156 is formed of one ormore metals or alloys, such as stainless steel. The base 156 canalternatively be formed of various different combinations of theabove-noted polymers and metals. The base 156 can be formed using any ofvarious different techniques, including machining, molding, and castingtechniques.

Referring again to FIGS. 5 and 6, fluid line connectors 160 arepositioned along the bottom edge of the cassette 112. The fluid pathways158 in the cassette 112 lead from the pumping chambers 138A, 138B to thevarious connectors 160. The connectors 160 are positioned asymmetricallyalong the width of the cassette 112. The asymmetrical positioning of theconnectors 160 helps to ensure that the cassette 112 will be properlypositioned in the cassette compartment 114 with the membrane 140 of thecassette 112 facing the cassette interface 110. The connectors 160 areconfigured to receive fittings on the ends of the dialysis solution baglines 126, the heater bag line 128, the patient line 130, and the drainline 132. One end of the fitting can be inserted into and bonded to itsrespective line and the other end can be inserted into and bonded to itsassociated connector 160. By permitting the dialysis solution bag lines126, the heater bag line 128, the patient line 130, and the drain line132 to be connected to the cassette, as shown in FIGS. 1 and 2, theconnectors 160 allow dialysis solution to flow into and out of thecassette 112 during use.

As shown in FIG. 9, before treatment, the door 108 of the PD cycler 102is opened to expose the cassette interface 110, and the cassette 112 ispositioned with its membranes 140, 161A, 161B adjacent to the cassetteinterface 110. The cassette 112 is positioned such that the pumpchambers 138A, 138B of the cassette 112 are aligned with the pistonheads 134A, 134B of the pistons 135A, 135B. In order to ensure that thepump chambers 138A, 138B align with the piston heads 134A, 134B, thecassette 112 is positioned between the locating pins 148 and the lowerledge 150 extending from the cassette interface 110. The asymmetricalpositioning of the connectors 160 of the cassette 112 act as a keyingfeature that reduces or eliminates the likelihood that the cassette 112will be installed with the membranes 140, 161A, 161B facing in the wrongdirection (e.g., facing outward toward the door 108). Additionally oralternatively, the locating pins 148 can be dimensioned to be less thanthe maximum protrusion of the projections 154A, 154B such that thecassette 112 cannot contact the locating pins 148 if the membranes 140,161A, 161B are facing outward toward the door 108.

While loading the cassette 112 into the PD cycler 102, the piston heads134A, 134B are typically retracted within the piston access ports 136A,136B. This positioning of the piston heads 134A, 134B can reduce thelikelihood of damage to the piston heads 134A, 134B during installationof the cassette 112.

FIGS. 10A-10B illustrate the pump chamber 138A and its associated pistonhead 134A throughout different phases of operation. It should beunderstood that the other piston head 134B would operate in a similarmanner to pump dialysis solution to and from the other pump chamber138B. During operation, the piston heads 134A, 134B are reciprocated tosequentially alter the volume of each of the pump chambers 138A, 138B.Typically, as the piston head 134A is extended (or stroked outwardly),the other piston head 134B is retracted (or stroked inwardly), and viceversa. As a result, dialysis solution is expelled from the pump chamber138A at the same time that dialysis solution is drawn into the pumpchamber 138B, and vice versa.

Referring to FIG. 10A, with the cassette 112 positioned adjacent to thecassette interface 110, the door 108 of the PD cycler 102 is closed overthe cassette 112 such that the cassette 112 is contained within thecassette compartment 114 between the door 108 and the cassette interface110. With the cassette 112 positioned in the cassette compartment 114,the inflatable pad within the door 108 is inflated to compress thecassette 112 between the door 108 and the cassette interface 110. Thiscompression of the cassette 112 holds the projections 154A, 154B of thecassette 112 in the recesses 152A, 152B of the door 108 and presses themembrane 140 tightly against the raised ridges 167 extending from theplanar surface of the rigid base 156 to form the enclosed fluid pathways158 and dome regions 146 (shown in FIG. 5). Because the PD system 100does not require a vacuum system to move the pumping membranes 161A,161B overlying the pump chambers 138A, 138B, a substantially airtightseal between the door 108 and the cassette interface 110 is typicallynot required. Thus, as compared to systems including a vacuum systemadapted to retract portions of the cassette membrane overlying pumpchambers, the door sealing mechanism of the PD cycler 102 can be simplerand more cost effective.

As shown in FIG. 10B, with the cassette 112 secured within the cassettecompartment 114, the piston 135A is advanced toward the base 156 of thecassette 112. As the piston 135A is advanced to its substantially fullyextended position, the piston head 134A pushes the dome-shaped portion179A of the pumping membrane 161A into the recessed region 163A of thebase 156 such that the dome-shaped portion 179A of the pumping membrane161A becomes inverted. In this position, the inner surface of thedome-shaped portion 179A of the pumping membrane 161A comes into contactor near contact with the inner surface of the hemispherical projections154A, 154B of the rigid base 156 of the cassette 112. As a result, thevolume of the pump chamber 138A is at its minimum such that all ornearly all liquid that was present in the pump chamber 138A prior toadvancement of the piston 135A is expelled from the pump chamber 138Aand into the fluid passageways 158 of the cassette 112 that lead awayfrom the pump chamber 138A as the piston 135A is advanced into thisfully extended position.

In order to draw PD solution into the pump chamber 138A, the piston 135Ais then retracted to a retracted position, as shown in FIG. 10C. As thepiston head 134A retracts, the resilient dome-shaped portion 179A of thepumping membrane 161A automatically rebounds (or self-expands) toincrease the volume of the pump chamber 138A. While the retraction speedof the piston head 134A will limit the speed with which the dome-shapedportion of the pumping membrane 161A is allowed to rebound, the pistonhead 134A does not apply a pulling force to the pumping membrane 161A.Instead, the inherent resiliency of the dome-shaped portion 179A of thepumping membrane 161A is what causes the dome-shaped portion 179A torebound outwardly away from the recessed region 163A of the base 156 ofthe cassette 112. The dome-shaped portion 179A of the pumping membrane161A is not allowed to rebound faster than the piston head 134A isretracted because the piston head 134A would apply a resistance force tothe dome-shaped portion 179A of the pumping membrane 161A in thatsituation. As a result, the retraction speed of the piston head 134A canbe controlled in a manner to ensure that vacuum pressure generatedwithin the pump chamber 138A is maintained within an acceptable range.

After retracting the piston head 134A a desired distance, the dialysisfluid in the pump chamber 138A can be forced out of the pump chamber138A by again returning the piston head 134A to the fully extendedposition shown in FIG. 10B, causing the pumping membrane 161A to deflectinward toward the rigid base 156 and thus decreasing the volume of thepump chamber 138A.

As noted above, while forcing dialysis solution into and out of the pumpchambers 138A, 138B, certain inflatable members 142 of the PD cycler 102can be selectively inflated to direct the pumped dialysis solution alongdesired pathways in the cassette 112.

Referring back to FIGS. 1 and 2, during PD treatment, the patient line130 is connected to a patient's abdomen via a catheter, and the drainline 132 is connected to a drain or drain receptacle. The PD treatmenttypically begins by emptying the patient of spent dialysis solution thatremains in the patient's abdomen from the previous treatment. To dothis, the motors of the PD cycler 102 are activated to cause the pistons135A, 135B to reciprocate and selected inflatable members 142 areinflated to cause the spent dialysis solution to be drawn into the pumpchambers 138A, 138B of the cassette 112 from the patient and then pumpedfrom the pump chambers 138A, 138B to the drain via the drain line 132.This flow path of the spent dialysis solution through the fluid pathways158 in the cassette 112 is shown in FIG. 11A.

After draining the spent dialysis solution from the patient, heateddialysis solution is transferred from the heater bag 124 to the patient.To do this, the motors of the PD cycler 102 are activated to cause thepistons 135A, 135B to reciprocate and certain inflatable members 142 ofthe PD cycler 102 are inflated to cause the spent dialysis solution tobe drawn into the pump chambers 138A, 138B of the cassette 112 from theheater bag 124 via the heater bag line 128 and then pumped from the pumpchambers 138A, 138B to the patient via the patient line 130. This flowpath of the dialysis solution through the fluid pathways 158 in thecassette 112 is shown in FIG. 11B.

Once the dialysis solution has been pumped from the heater bag 124 tothe patient, the dialysis solution is allowed to dwell within thepatient for a period of time. During this dwell period, toxins cross theperitoneum into the dialysis solution from the patient's blood. As thedialysis solution dwells within the patient, the PD cycler 102 preparesfresh dialysate for delivery to the patient in a subsequent cycle. Inparticular, the PD cycler 102 pumps fresh dialysis solution from one ofthe four full dialysis solution bags 122 into the heater bag 124 forheating. To do this, the motors of the PD cycler 102 are activated tocause the pistons 135A, 135B to reciprocate and certain inflatablemembers 142 of the PD cycler 102 are inflated to cause the dialysissolution to be drawn into the pump chambers 138A, 138B of the cassette112 from the selected dialysis solution bag 122 via its associated line126 and then pumped from the pump chambers 138A, 138B to the heater bag124 via the heater bag line 128. This flow path of the dialysis solutionthrough the fluid pathways 158 in the cassette 112 is shown in FIG. 11C.

After the dialysis solution has dwelled within the patient for thedesired period of time, the spent dialysis solution is pumped from thepatient to the drain. The heated dialysis solution is then pumped fromthe heater bag 124 to the patient where it dwells for a desired periodof time. These steps are repeated with the dialysis solution from two ofthe three remaining dialysis solution bags 122. The dialysis solutionfrom the last dialysis solution bag 122 is typically delivered to thepatient and left in the patient until the subsequent PD treatment.

While the dialysis solution has been described as being pumped into theheater bag 124 from a single dialysis solution bag 122, dialysissolution can alternatively be pumped into the heater bag 124 frommultiple dialysis solution bags 122. Such a technique may beadvantageous, for example, where the dialysis solutions in the bags 122have different concentrations and a desired concentration for treatmentis intermediate to the concentrations of the dialysis solution in two ormore of the bags 122.

After completion of the PD treatment, the piston heads 134A, 134B areretracted away from the cassette 112 to a sufficient distance such thatthe piston heads 134A, 134B no longer contact the pumping membranes161A, 161B. The door 108 of the PD cycler is then opened and thecassette 112 is removed from the cassette compartment and discarded.

While certain implementations have been described, other implementationsare possible.

While the above-described cassette 112 includes the support rings 169A,169B and the retainer rings 171A, 171B, which are positioned on oppositesides of the base 156 from one another and are drawn together usingfasteners in order to compress the pumping membranes 161A, 161B againstthe base 156, other techniques for securing the rings to one another canbe used. For example, the rings can be ultrasonically staked or weldedto one another.

While the pumping membranes 169A, 169B have been described as includingencapsulated support rings 169A, 169B, in some implementations, thepumping membranes are separate components from the support rings. Insuch implementations, for example, the peripheral regions of the pumpingmembranes can be placed against portions of the cassette basesurrounding the recessed regions that form the pump chambers, and thesupport rings can be positioned against the outer surfaces of theperipheral regions of the pumping membranes. The retainer rings can bepositioned on the opposite side of the base from the pumping membranesin much the same way as described above, and the associated supportrings and retainer rings can be secured together using any of thetechniques described above to compress the sandwiched peripheral regionof the pumping membrane against the cassette base to provide aliquid-tight seal around the pump chamber.

While the above-described cassettes have been described as using supportrings and retainer rings to compress the pumping membranes against thebase of the cassette, in certain implementations, only support rings areused. In such implementations, for example, the support rings, which canbe either encapsulated within the pumping membranes or positionedagainst the outer surfaces of the pumping membranes, are secured to thebase of the cassette rather than being secured to retainer rings on theopposite side of the base of the cassette. The support rings can besecured to the base using fastener elements, ultrasonic staking orwelding, or any of various other suitable techniques described herein.

FIGS. 12 and 13 are perspective views from a membrane side and from arigid base side of a cassette 212. The cassette 212 includes a rigidbase 256 that is similar to the rigid bases of the cassettes describedabove except for the location of apertures through which bolts 275extend differ from those cassettes described above. A support ring 269in the shape of a figure eight is used to compress peripheral regions ofpumping membranes 261A,261B against portions of the base 256 thatsurround recessed regions of the base 256 in order to provideliquid-tight fluid pump chambers between the pumping membranes 261A,261B and the recessed regions of the base 256. Unlike the pumpingmembranes 161A, 161B described above, which include encapsulated supportrings 169A, 169B, the support ring 269 and pumping membranes 261A, 261Bof the cassette 212 shown in FIGS. 12 and 13 are separate components. Asshown in FIG. 12, the support ring 269 is placed against the outersurfaces of the peripheral regions of the pumping membranes 261A, 261B.The support ring 269 is shaped to conform to the various raised featuresextending from the base 256 of the cassette 212. This helps to ensurethat the support ring 269 can be pulled tightly enough against the base256 to form the liquid-tight seal between the peripheral regions of thepumping membranes 261A, 261B and the portions of the base 256surrounding the pumping chambers 238A, 238B. Typically, the support ringhas a thickness of about 0.050 inch to about 0.200 inch.

Unlike certain cassettes described above, the cassette 212 does notutilize a retainer ring. Rather, as shown in FIG. 13, heads of the bolts275, which pass through aligned apertures in the support ring 269, thepumping membranes 261A, 261B, and the base 256, directly contact theouter surface of the rigid base 256. Thus, while the nuts 273 aresupported by the support ring 269, the base 256 provides direct supportfor the heads of the bolts 275. Compression forces generated by screwingthe nuts 273 onto the bolts 275 are used to compress the peripheralregions of the pumping membranes 261A, 261B between the support ring 269and the rigid base 256.

The pumping membranes 261A, 261B and the support ring 269 can be formedof any of the materials described above with respect to the pumpingmembranes 161A, 161B and the support rings 169A, 169B, respectively.

While the cassettes described above include one or more rings that areused to secure the pumping membranes to the cassette base, in certainimplementations, cassettes are constructed with no such rings. In suchimplementations, for example, the pumping membrane of the cassette canbe welded or adhesively secured to the flange surrounding the recessedregion of the base.

While the cassettes described above include pumping membranes that havedome-shaped central regions, in certain implementations, substantiallyflat or planar pumping membranes are used. The flat pumping membranescan be formed of any of the various materials used to form the pumpingmembranes described above and can be secured to the base of the cassetteusing any of the various techniques described above. The pumpingmembranes typically have a thickness of at least 0.125 inch. Forexample, the flat pumping membranes can have a thickness of about 0.125inch to about 0.250 inch. Using relatively thick pumping membranes (ascompared to membranes used on many conventional cassettes), helps toensure that the pumping membranes are resilient and able to rebound (orself-expand) when depressed into the pump chamber and then released.

In addition to attaching the pumping membranes to the base, the pumpingmembranes can include resilient o-rings around their outercircumferences. The o-rings can be stretched around annular projectionsextending form the cassette base in order to further secure the pumpingmembranes to the base. The engagement between the annular projections ofthe base and the o-rings of the pumping membranes can help to stabilizethe circumferential regions of the pumping membranes to ensure thatthose circumferential regions do not slip into the recessed regions ofthe base that form the pump chambers as the piston heads advance centralportions of the pumping membranes into the recessed regions.

While the pumping membranes have been described as substantiallycircular membranes that are positioned over the pump chambers, incertain implementations, a pumping membrane is sized and shaped to coversubstantially the entire base of the cassette. In such implementations,the pumping membrane can be attached to a peripheral region of the base.The pumping membrane can also be attached to portions of the base thatsurround the pump chambers. The pumping membrane is substantially flator planar and can be formed of the same material(s) as the flatmembranes discussed above. The pumping membrane can have the samethickness as the flat membranes discussed above. In someimplementations, the portions of the pumping membrane overlying the pumpchambers have a greater thickness than the surrounding portions of themembrane. For example, the portions of the pumping membrane overlyingthe pump chambers can have a thickness of about 0.125 inch to about0.250 inch, while the other portions of the pumping membrane have athickness of about 0.004 inch to about 0.006 inch.

While the cassettes described above include pumping membranes that aresecured to and directly contact the rigid base of the cassette, in someimplementations, the pumping membranes are attached to an outer surfaceof another flexible membrane that overlies the pump chambers of thecassette. As shown in FIG. 14, for example, a cassette 312 includes atray-like rigid base 356 and a flexible membrane 340 that is attached to(e.g., thermally bonded to, adhered to) the periphery of the base 356and covers substantially the entire surface area of the base 356. Themembrane 340 cooperates with recessed regions of the base 356 to formpump chambers 338A, 338B and cooperates with raised structural features367 extending from planar regions of the base 356 to form a series offluid pathways 358 and multiple, depressible dome regions 346, which arewidened (e.g., substantially circular) portions of the fluid pathways358.

Pumping membranes 361A, 361B are attached to (e.g., thermally bonded to,adhered to) those portions of the membrane 340 that overlie the pumpchambers 338A, 338B. Typically, substantially the entire surface areasof the pumping membranes 361A, 361B are attached to the correspondingsurfaces areas of the flexible membrane 340 to ensure that the portionsof the flexible membrane 340 overlying the pump chambers 338A, 338B movein tandem with the pumping membranes 361A, 361B. The pumping membranes361A, 361B are more resilient than the flexible membrane 340. Thus, thepumping membranes 361A, 361B can provide the flexible membrane walls ofthe pump chambers 338A, 338B with greater resiliency such that theflexible membrane walls can rebound under their own force (i.e., withoutthe piston pulling the flexible membrane wall) to create vacuum pressurewithin the pump chambers 338A, 338B and draw fluid into the pumpchambers 338A, 338B. The flexible membrane 340 can be identical to theflexible membrane 140 described above except the flexible membrane 340does not include cut-outs over the pump chambers 338A, 338B. The pumpingmembranes 361A, 361B can be formed of the same material(s) as thepumping membranes discussed above. The pumping membranes typically havea thickness about 0.125 inch to about 0.250 inch.

As an alternative to using thermal or adhesive bonds betweensubstantially the entire surface areas of the pumping membranes 361A,361B and the flexible membrane 340, in certain implementations, only theperimeter edge regions of the pumping membranes 361A, 361B are bonded tothe flexible membrane 340. In such implementations, for example, any airbetween the pumping membranes 361A, 361B and the flexible membrane 340can be evacuated prior to forming the perimeter bond around the pumpingmembranes 361A, 361B. As a result, a passive vacuum between the pumpingmembranes 361A, 361B and the flexible membrane 340 will ensure thatthose portions of the flexible membrane 340 overlying the pump chambers338A, 338B will move in tandem with the pumping membranes 361A, 361B.

While the pumping membranes 361A, 361B have been described assubstantially circular membranes that are attached to the portions ofthe membrane 340 overlying the pump chambers 338A, 338B, in certainimplementations, a single pumping membrane is sized and shaped to coversubstantially the entire base 356 of the cassette 312. In suchimplementations, the pumping membrane is typically attached tosubstantially the entire surface area of the membrane 340. The pumpingmembrane is typically substantially flat or planar and can be formed ofthe same material(s) and have the same thickness as the pumpingmembranes 361A, 361B discussed above. Because the pumping membrane isrelatively thick and thus requires significant force to deflect, thepumping membrane can include cutouts that align with the dome regions346 of the cassette. In this manner, the inflatable valve members of thePD cycler can generate sufficient force to depress the exposed portionsof the membrane 340 adjacent the cutouts and prevent fluid flow throughthe associated fluid passageway of the cassette. Alternatively, the PDcycler can be modified to generate increased pressures within theinflatable members that are sufficient to press the thicker pumpingmembrane and the underlying membrane 340 against the base 356 of thecassette to control liquid flow therethrough.

While the cassettes discussed above have been described as having twopump chambers, the cassettes can alternatively have more or fewer thantwo pump chambers.

While each of the pump chambers of the cassettes described above hasbeen described or illustrated as including a fluid inlet port and afluid outlet port, the pump chambers can alternatively include a singleport that is used as both an inlet and an outlet.

While certain cassettes have been described as being positioned betweenlocating pins and a lower ledge extending from a cassette interface ofthe PD cycler in order to hold the cassette in a position such that thepiston heads align with the pump chambers of the cassette, othertechniques for ensuring that the piston heads align with the pumpchambers can alternatively or additionally be used. In someimplementations, for example, the cassette is placed against the door ofthe PD cycler with the hollow projections of the cassette disposed inrecesses of the PD cycler's door. The cassette is held in this positionby retainer clips attached to the door. Upon closing the door, thepiston heads of the PD cycler align with the pump chambers of thecassette.

While pistons having substantially rigid piston heads have beendescribed, pistons having piston heads with compressible portions canalternatively be used. As shown in FIG. 15, for example, a piston 335Aincludes a piston head 334A secured to the piston shaft 133A. The pistonhead 334A includes a rigid core 336A surrounded by a compressiblecoating 341A. The rigid core 336A can be formed of any of the variousmaterials described above with respect to the piston heads 134A, 134B.The compressible coating 341A is typically formed of an elastomer, suchas Silicone, Medalist®, and thermoplastic elastomers (e.g., DYNAFLEXG2700, DYNAFLEX G6700, etc.).

As shown in FIG. 15, the compressible coating 341A increases inthickness, as measured in the axial direction of the piston head 334A,from a leading tip 337A of the piston head 334A towards an outercircumferential edge region 339A of the piston head 334A at which themaximum diameter of the piston head 334A can be found. As a result, thevast majority of the thickness of the piston head 334A (measured in theaxial direction of the piston head 334A) in the outer circumferentialedge region 339A is made up of the flexible elastomeric material of thecompressible coating 341A, while the vast majority of the thickness ofthe piston head 334A (measured in the axial direction of the piston head334A) in the tip region of the piston head 334A is made up of the rigidmaterial of the core 336A. In some implementations, the compressiblecoating 341A has a thickness of about 0.1 inch to about 0.2 inch at thetip 337A of the piston head 334A and has a thickness of about 0.25 inchto about 0.45 inch at the outer circumferential edge region 339A of thepiston head 334A. The compressible coating 338A increases the diameterof the piston head 334A (as compared to the piston heads 134A, 134Bdescribed above) at the leading edge of the piston head 334A and thusincreases the surface area of the piston head 334A that contacts thepumping membrane of the cassette at a given time during use. As aresult, the force applied to the pumping membrane of the cassette duringuse can be more uniformly distributed over the surface area of theportion of the pumping membrane overlying the pump chamber.

FIGS. 16A-16D schematically illustrate a method of using a PD cyclerequipped with the piston 335A of FIG. 15 to expel dialysis solution fromand draw dialysis solution into the fluid pump chamber 338A of thecassette 312 illustrated in FIG. 14. It should be understood that whileonly one piston 335A and one pump chamber 338A are illustrated in FIGS.16A-16D, the PD cycler would generally be equipped with two pistons335A, 335B that are aligned with the two corresponding pump chambers338A, 338B of the cassette 312. Both of the pistons 335A, 335B would beoperated in generally the same manner to pump dialysis solution to andfrom their respective pump chambers 338A, 338B.

Referring to FIG. 16A, the cassette 312 is first positioned in acassette compartment 314 formed between the door 108 and the cassetteinterface 110 of the PD cycler and then compressed between the door 108and the cassette interface 110 in the manner described above. Thiscompression of the cassette 312 presses the pumping membrane 361A andthe flexible membrane 340 against the base 356 to form a liquid-tightseal around the pump chamber 338A and also presses the membrane 340tightly against the raised ridges 367 extending from the planar surfaceof the rigid base 356 to form the enclosed fluid pathways 358 and domeregions 346.

As shown in FIG. 16B, with the cassette 312 secured within the cassettecompartment 314, the piston head 334A is advanced into contact with thepumping membrane 361A. Due to the compressible coating 338A positionedaround the core 336A of the piston head 334A, which increases thediameter of the leading surface of the piston head 334A and makes thediameter of the piston head 334A more uniform along the length of thepiston head 334A (as compared to the piston heads 134A, 134B describedabove), a larger area of the piston head 334A contacts the pumpingmembrane 361A during the initial phases of the outward stroke of thepiston 335A. The leading surface of the piston head 334A can, forexample, contact substantially the entire surface area of the portion ofthe pumping membrane 361A overlying the pump chamber 338A. In someimplementations, for example, the piston head 334A, upon initial contactwith the pumping membrane 361A, is in contact with at least 90 percent(e.g., at least 95 percent) of the surface area of the portion of thepumping membrane 361A that overlies the pumping chamber 338A. As aresult, the pressure applied to the pumping membrane 361A by the pistonhead 334A is more uniformly distributed over the surface area of thepumping membrane 361A.

As the piston head 334A is further advanced, the pumping membrane 361Aand the underlying portion of the membrane 340 are pushed into arecessed region 363A of the base 356 of the cassette 312, which reducesthe volume of the pump chamber 338A and increases the fluid pressurewithin the pump chamber 338A. Such increased pressures within pumpchambers can cause portions of membranes that overlie the pump chambersand are not in contact with the piston head to bulge outward if thosemembranes do not have sufficient strength to resist that pressure. Byincreasing the diameter of the leading portions of the piston head 334A,the surface area of the pumping membrane 361A contacted by the pump head334A during the initial phases of the outward stroke of the pump head334A is increased. As a result, the tendency of the flexible membrane340 and the pumping membrane 361A to bulge outward due to increasedfluid pressure within the pump chamber 338 is decreased. Additionally,the thickness of the combination of the flexible membrane 340 and thepumping membrane 361, which is greater than the thickness of manyconventional fluid pumping cassette membranes, helps the flexiblemembrane 340 and the pumping membrane 361A to withstand the increasedfluid pressure within the pump chamber 338A without bulging outward.

As shown in FIG. 16C, when the piston head 334A is fully extended, thecompressible coating 341A is compressed to substantially conform to theshape of the recessed region 363A of the rigid base 156 of the cassette112. In this position, the volume of the pump chamber 338A is at itsminimum such that all or nearly all liquid that was present in the pumpchamber 338A prior to advancement of the piston 335A will be expelledfrom the pump chamber 338A and into the fluid passageways 358 of thecassette 312 as the piston 335A is advanced into this fully extendedposition.

The leading portions of the piston head 334A that extend farthest intothe recessed region 363A, when in an uncompressed state, have a greaterdiameter than the diameter of the portions of the of the recessed region363A that those leading portions of the piston head 334A contact in thisfully extended position. As the piston head 334A is advanced into itsfully extended position, the resistance of the pumping membrane 361Aand/or contact with the recessed region 363A of the base 356 causes thecompressible coating 341A in those leading portions of the piston head334A to compress. This compression of the coating 341A decreases thediameter of the leading portions of the piston head 334A and allows thepiston head 334A to be fully received within the recessed region 363A ofthe base 312. In this way, the piston head 334A provides a sufficientcontact area with the pumping membrane 361A during the initial phases ofthe outward stroke to prevent outward bulging of the flexible membrane340 and the pumping membrane 361A, while also being capable of beingfully inserted into the pump chamber 338A to ensure that the desiredvolume of fluid is expelled from the pump chamber 338A. Because outwardbulging of the membranes 340, 361A is eliminated or at leastsignificantly reduced, the pumping volume accuracy can be increased.

In order to draw PD solution into the pump chamber 338A, the piston head334A is then retracted to a retracted position, as shown in FIG. 16D. Asthe piston head 334A retracts, the resilient pumping membrane 361Aautomatically rebounds (or self-expands) and pulls the portion of theflexible membrane 340 underlying the pump membrane 338A with it toincrease the volume of the pump chambers 338A. This creates vacuumpressure within the pump chamber 338A and thus draws the PD solutioninto the pump chamber 338A.

After the pumping membrane 361A has rebounded to a sufficient extent todraw a desired volume of the PD solution into the pump chamber 338A, thedialysis fluid in the pump chamber 338A can be forced out of the pumpchamber 338A by again returning the piston head 334A to the fullyextended position shown in FIG. 16C, causing the pumping membrane 361Ato deflect inward toward the rigid base 356 and thus decreasing thevolume of the pump chamber 338A.

While forcing dialysis solution into and out of the pump chambers 338A,338B, inflatable members of the PD cycler that align with the domeregions 346 of the cassette 312 can be selectively inflated to directthe pumped dialysis solution along desired pathways in the cassette 312in order to carry out the PD treatment.

While the piston 335A has been described as being used with the cassette312 of FIG. 14, it should be understood that the piston 335A can be usedwith any of the various other cassettes described herein. In addition,the piston 335A can be used with cassettes having only a thin,single-layer membrane over the pump chamber. An example of such acassette would be the cassette of FIG. 14 but with the pumping membranes361A, 361B removed. The piston 335A can be particularly useful withthose types of cassettes due to the propensity of the thin, single-layermembrane to bulge outward in regions of the membrane that overlie thepump chamber when fluid pressures within the pump chamber increases.

While the piston head 334A has been described as having a core aboutwhich a compressible coating is disposed, other types of piston headscan be used. For example, FIG. 17 is a cross-sectional view of analternative piston 435A that includes a piston head 434A that is formedof an elastomer. Any of the elastomers described above as being suitablefor forming the compressible coating 341A of the piston head 334A can beused to form the piston head 434A. The thickness of the piston head 434Agradually decreases from a central portion 437A of the piston head 434Ato an outer circumferential region 439A of the piston head 434A. Thecentral portion 437A of the piston head 434A can, for example, be about0.2 inch to about 0.55 inch thicker than the circumferential edge region439A of the piston head 434A. In some implementations, the centralportion 437A of the piston head 434A has a thickness of about 0.45 inchto about 0.65 inch and the circumferential edge region 439A of thepiston head 434A has a thickness of about 0.1 inch to about 0.25 inch.Due to the decreased thickness of the elastomeric material near theouter circumference of the piston head 434A, the circumferential edgeregion 439A of the piston head 434A is more flexible than the centralportion 437A of the piston head 434A.

The piston head 434A behaves in a manner similar to the piston head 334Aillustrated in FIGS. 16A-16D when it is used to pump fluid out of anddraw fluid into a pump chamber of any of the various cassettes describedherein. In particular, due to the size and shape of the piston head434A, the piston head 434A will contact a large surface area of thecassette membrane overlying the pump chamber to reduce the likelihood ofouter circumferential regions of that membrane bulging outwardly as thepiston head 434A is advanced to expel fluid from the pump chamber. Asthe piston head 434A is advanced further into the recessed region of thecassette base that forms the pump chamber, the resistance of themembrane and/or rigid cassette base itself will cause the outer portionsof the piston head 434A to flex and ultimately conform to the generalshape of the recessed region.

FIGS. 18A and 18B are cross-sectional views of a piston 535A having apiston head 534A that includes multiple concentric rings 538A, 540A,542A secured to a leaf spring 544A. A cylindrical member 536A isattached to the leading end of the piston shaft 133A and is positionedat the center point of the concentric rings 538A, 540A, 542A. The leafspring 544A is also attached to the piston shaft 133A. In someimplementations, the leaf spring 544A includes an opening that receivesthe piston shaft 133A and the leaf spring 544A is welded to the pistonshaft 133A around the circumference of that opening. However, othertechniques, such as mechanical fastening techniques, can alternativelyor additionally be used to secure the leaf spring 544A to the pistonshaft 133A. The leaf spring 544A is biased to a flat or planarconfiguration, as shown in FIG. 18A. Thus, when no axial forces arebeing applied to the rings 538A, 540A, 542A of the piston head 534A, thepiston head 534A will have a substantially flat front surface, as shownin FIG. 18A.

The piston head 534A can be used with any of the various cassettesdescribed herein to pump fluid from and draw fluid into a pump chamberof the cassette. Due to the flat front face of the piston head 534A ofthe piston 535A, the piston 535A is particularly beneficial for use withthe cassettes described herein that include one or more flat membranesthat overlie the pump chamber. As the piston head 534A is advanced intocontact with such a membrane, the front surface of the piston head 534Awill contact substantially the entire surface area of the portion of themembrane overlying the pump chamber. As a result, a substantiallyuniform pressure will be applied over that portion of the membrane. Asthe piston head 534A is advanced into the recessed region of the basethat forms the pump chamber, the outer ring 542A will contact therecessed region of the cassette base causing the leaf spring 544A toflex slightly. Upon further advancement of the piston head 534A, thenext outermost ring 540A will contact the recessed region of thecassette base causing the leaf spring 544A to flex further. When thepiston head 534A has been fully advanced into the recessed region of thecassette base, each of the concentric rings 538A, 540A, 542A will be incontact (through the cassette membrane) with the recessed region of thecassette base and the piston head 534A will generally conform to theshape of the recessed region, as shown in FIG. 18B.

While the leaf spring 544A has been described as being attached to thepiston shaft 133A, the leaf spring 544A can alternatively oradditionally be attached to the cylindrical member 536A on the end ofthe piston shaft 133A. The leaf spring 544A can, for example, be weldedor mechanically fastened to the cylindrical member 536A.

While the concentric rings 538A, 540A, 542A and the cylindrical member536A of the piston head 534A have been described as being exposed oropen, in some implementations, an elastomeric cover or coating isapplied to the front face of the piston head 534A. Alternatively, theentire piston head 534A can be encapsulated within such an elastomericcoating. The coating can help to provide a more gradual transition fromone concentric ring to the next when the piston head 534A is advancedinto the recessed region of the cassette base. Thus, the coating canhelp to ensure that a uniform force is applied to the portion of themembrane overlying the pump chamber throughout the pumping process,which can improve the pumping volume accuracy of the PD cycler.

While the piston head 534A has been described as including only threeconcentric rings, in certain implementations, more concentric rings areused to form the piston head. It will be appreciated that the use ofmore concentric rings that are smaller will provide a less abrupttransition from one ring to the next as the rings are deflected.

As an alternative to the types of compressible or collapsible pistonheads described above, a piston head that includes overlapping segmentsthat can collapse under one another can be used. As shown in FIGS. 19Aand 19B, for example, a piston 635A has a piston head 634A that includesmultiple overlapping leaves 636A that can transition from an expandedposition (shown in FIG. 19A) to a contracted position (shown in FIG.19B). The overlapping leaves 636A are all connected to one another via apin 638A at the apex of the piston head 634A. Additionally, at pointsnearer the circumferential edge of the piston head 634A, adjacent leaves636A are connected to one another via springs that bias the leaves 636Ato the expanded position (shown in FIG. 19A) in which the leaves 636Aonly minimally overlap one another. As the piston head 634A is advancedinto the recessed region of the cassette base during use, the forceapplied to the leaves 636A due to contact with the membrane and thecassette base exceeds the spring force of the spring and causes theoverlapping leaves 636A to collapse under one another. As a result, thediameter of the piston head 634A decreases, allowing the piston head634A to generally conform to the shape of the recessed region of thecassette base that forms the pump chamber (shown in FIG. 19B). Becausethe piston head 634A is larger than the recessed region of the cassettebase, the area of the membrane contacted by the piston head 634A can begreater during the initial phases of the outward stroke of the piston635A than conventional rigid piston heads, which are generally sized andshaped to match the size and shape of the recessed region of thecassette base. As a result the force of the piston head 634A can be moreuniformly applied to the membrane resulting in less bulging of themembrane and greater pumping volume accuracy.

While certain PD cyclers above have been described as including a touchscreen and associated buttons, the PD cycler can include other types ofscreens and user data entry systems. In certain implementations, forexample, the cycler includes a display screen with buttons (e.g.,feathertouch buttons) arranged on the console adjacent the displayscreen. Certain buttons can be arranged to be aligned with operationaloptions displayed on the screen during use such that the user can selecta desired operational option by pressing the button aligned with thatoperational option. Additional buttons in the form of arrow buttons canalso be provided to allow the user to navigate through the variousdisplay screens and/or the various items displayed on a particularscreen. Other buttons can be in the form of a numerical keypad to allowthe user to input numerical values in order, for example, to inputoperational parameters. A select or enter button can also be provided toallow the user to select an operational option to which the usernavigated by using the arrow keys and/or to allow the user to entervalues that the user inputted using the numerical keypad.

While the doors of the PD cyclers described above are shown as beingpositioned on a front face of the PD cyclers, the doors canalternatively be positioned at various other locations on the PDcyclers. For example, the doors could be positioned on a top face of thePD cycler such that the cassette is slid into the cassette compartmentin a substantially horizontal orientation instead of a substantiallyvertical orientation.

While some of the PD cyclers discussed above have been described asincluding inflatable pads in their doors to compress the cassettebetween the door and the cassette interface, the PD cyclers canalternatively or additionally include inflatable pads positioned behindthe cassette interface.

While the cassettes described above have been described as being part ofa PD system, these types of cassettes can be used in any of variousother types of cassette-based medical fluid pumping systems. Otherexamples of medical fluid pumping systems with which cassettes describedherein can be used include hemodialysis systems, blood perfusionsystems, and intravenous infusion systems.

Similarly, while the cassettes have been described as being used to pumpdialysis solution, other types of dialysis fluids can be pumped throughthe cassettes. As an example, in the case of cassettes used withhemodialysis machines, blood can be pumped through the cassettes. Inaddition, priming solutions, such as saline, can similarly be pumpedthrough cassettes using the various different systems and techniquesdescribed above. Similarly, as an alternative to dialysis fluids, any ofvarious other types of medical fluids can be pumped through theabove-described cassettes depending on the type of medical fluid pumpingmachines with which the cassettes are used.

What is claimed is:
 1. A medical fluid pumping system, comprising: amedical fluid pumping machine defining a cassette enclosure, the medicalfluid pumping machine comprising a movable piston; and a medical fluidcassette configured to be disposed within the cassette enclosure of themedical fluid pumping machine, the medical fluid cassette comprising abase having a first region and a second region; a first membraneoverlying the first region of the base on a first side of the base, thefirst membrane cooperating with the first region of the base to form atleast one fluid pathway; and a second membrane overlying the secondregion of the base on the first side of the base, the second membranecooperating with the second region of the base to define a fluid pumpchamber, the second membrane being more resilient than the firstmembrane, and the cassette being positionable within the cassetteenclosure of the medical fluid pumping machine so that the secondmembrane can be moved toward the base by the piston to decrease a volumeof the fluid pump chamber and, upon retraction of the piston, the secondmembrane can rebound to increase the volume of the pump chamber.
 2. Themedical fluid pumping system of claim 1, wherein the second region ofthe base is a recessed region of the base.
 3. The medical fluid pumpingsystem of claim 1, wherein the second membrane has a greater thicknessthan the first membrane.
 4. The medical fluid pumping system of claim 1,wherein the first and second membranes are formed of differentmaterials.
 5. The medical fluid pumping system of claim 1, wherein thesecond region of the base is a recessed region of the base, and thesecond membrane is sized to overlie a portion of the base that surroundsthe recessed region.
 6. The medical fluid pumping system of claim 5,wherein a fluid-tight seal is formed between a peripheral region of thesecond membrane and the base.
 7. The medical fluid pumping system ofclaim 6, wherein the second membrane comprises a dome-shaped portion. 8.The medical fluid pumping system of claim 6, wherein the medical fluidcassette further comprises a first ring that is secured to the secondmembrane and compresses the second membrane against the base.
 9. Themedical fluid pumping system of claim 8, wherein the first ring isfastened to the base.
 10. The medical fluid pumping system of claim 8,further comprising a second ring disposed on a side of the base oppositethe second membrane, the second ring secured to the first ring in amanner to cause the first ring to compress the second membrane againstthe base.
 11. The medical fluid pumping system of claim 6, wherein thesecond membrane is attached to the portion of the base that surroundsthe recessed region.
 12. The medical fluid pumping system of claim 1,wherein the first membrane defines an aperture that is aligned with thesecond region of the base, and the second membrane is at least partiallydisposed within the aperture of the first membrane.
 13. The medicalfluid pumping system of claim 1, wherein the first membrane coverssubstantially the entire surface of the base.
 14. The medical fluidpumping system of claim 13, wherein the second membrane is attached to aportion of the first membrane that overlies the second region of thebase.
 15. The medical fluid pumping system of claim 13, wherein thesecond membrane overlies substantially the entire surface of the base.16. The medical fluid pumping system of claim 15, wherein the secondmembrane comprises a plurality of cutouts that are aligned with valveregions of the cassette.
 17. The medical fluid pumping system of claim1, wherein the piston comprises a piston head having a circumferentialregion that is configured to move radially inward as the piston head ispressed against the second membrane to move the second membrane towardthe base.
 18. The medical fluid pumping system of claim 17, wherein thecircumferential region of the piston head is formed of an elastomericmaterial that compresses as the piston head is pressed against thesecond membrane to move the second membrane toward the base.
 19. Themedical fluid pumping system of claim 17, wherein the piston headcomprises a plurality of interleaved segments that move relative to oneanother to allow the circumferential region of the piston head tocollapse as the piston head is pressed against the second membrane tomove the second membrane toward the base.
 20. The medical fluid pumpingsystem of claim 17, wherein the piston head comprises a plurality oftelescoping segments that move relative to one another to allow thecircumferential region of the piston head to collapse as the piston headis pressed against the second membrane to move the second membranetoward the base.
 21. The medical fluid pumping system of claim 17,wherein at any given time throughout an outward stroke of the piston, anarea of a portion of the piston head in contact with the second membraneis substantially equal to an area of the pump chamber in a plane inwhich the second membrane lies.
 22. The medical fluid pumping system ofclaim 1, wherein the medical fluid pumping system is a dialysis system.23. The medical fluid pumping system of claim 22, wherein the medicalfluid pumping system is a peritoneal dialysis system.